1) Why is it not possible to use a 3rd card for PhysX if it is identical? (I know its not possible to Tri-SLI the 9800GT, I'm asking why its not possible to use the 3rd card, outside of the SLI for PhysX if it's an identical card.)

I stand corrected. I think it is possible for you to use the 3rd 9800GT for Physx. I apologize for causing your confusion.

1) If I had two 9800GT's in SLI, do I need to bridge a 3rd 9800GT in order to let it process PhysX, or can I just have it in the 3rd SLI slot, without having to connect it with the other two?

2) Can I have an ATI HD4870 as the main graphics card, and have a secondary Nvidia card to process PhysX, and PhysX only? (obviously if question one is not ruled out, and a bridge is not necessary.) Would this even work when it comes down to games that have PhysX enabled?

Thanks.

1, you don't need a SLI bridge for a Physx Card, but put on in anyway, the Tri SLI Bridges look badass!

2, yes you can, but the drivers are discontinued so your probably better off with Tri SLI 9800GT